Field of the Invention
New requirements which are placed on the brake systems of motor vehicles--such as, for example, anti-lock systems, drive stability closed-loop control systems, traction controllers, so-called intelligent cruise controls, etc.--have, together with the demand for a reduction in the assembly and maintenance costs, led to the development of new, purely electrically controlled and actuated brake systems (also referred to as brake-by-wire). In a prior art brake system of that kind, the wheel speeds of the individual wheels of the motor vehicle are measured with wheel sensors, and continuously acting wheel brake actuators are electrically controlled on an individual basis such that they apply an independent brake pressure to each wheel. See German published patent application DE 195 48 392 A1. A central control unit evaluates the sensor signals and carries out at the individual wheels a brake-pressure closed-loop control operation to prevent locking.
In another prior art brake system with hydraulically actuated wheel brakes and an individual closed-loop control system for two wheels which are arranged to the left and right of the direction of travel, in order to reduce the yawing moment when there are different coefficients of friction, the braking force at the wheel with the higher coefficient of friction is built up with a delay, kept constant, or reduced. See German published patent application DE 195 42 295 A1. The coefficients of friction are determined there by evaluating the wheel re-acceleration at the wheels after a wheel deceleration. By comparing the detected coefficients of friction at wheels on the right and left sides of the vehicle, it is determined which wheel has the higher coefficient of friction.
Another anti-lock system is described in U.S. Pat. No. 5,312,170 (DE 40 34 814 A1). That system has phases for adjusting to a setpoint slip value and phases for controlling the brake pressure in the case of instabilities of the wheel. The setpoint slip is determined from a detected, maximum coefficient of friction and the controlled brake pressure is determined from the slip value when the maximum coefficient of friction occurs. The anti-lock system is particularly suitable for trucks. The coefficient of friction, the maximum value of the coefficient of friction and also a slip value are determined there using very complex mathematical calculations and approximation methods.
A method of determining a frictional value between a tire and the road surface is described in U.S. Pat. No. 5,513,907 (European published patent application EP 0 630 786 A1). There, the coefficient of friction is determined by continuously measuring the vehicle speed, the rotational speed of a wheel, and the wheel brake pressure. With reference to the angular acceleration of the wheel which is determined from the wheel speed, and with reference to the brake pressure the coefficient of friction is determined as a function of the wheel slip by means of a recursive estimation algorithm and is used for more precisely closed-loop control of the anti-lock system.
A braked wheel should also be prevented from locking in the case of purely electrically actuated brake systems, that is to say they must be provided with an anti-lock system (ABS). Such a system has features in common with conventional anti-lock systems in hydraulic or partially hydraulic brake systems, but it differs from those systems in particular in terms of the actuators. In electrical brake systems, continuously operating actuation elements are used to apply the wheel brake pressure. However, for braking which is as effective as possible but free of locking, precise knowledge of the coefficient of friction between the tire and the underlying surface is important.